The present invention relates to a device and a method for application of coating powder in a screen or offset printing process. In particular, the invention relates to a device and a method for application of adhesive powder in a screen transfer printing process.
A conventional screen printing process includes applying one or more layers of color to an object, by pressing it through the mesh of a netting screen stretched on a frame, onto the object. A screen transfer printing process relates to producing a screen printed material (hereafter referred to as a screen transfer) that can be heat transferred to e.g. a garment. The material normally consists of several layers of colored ink having been applied to at least an area of a base sheet, i.e. a sheet of paper or cardboard. The coated area is then pulled of the sheet, and is ready to be attached to a garment or any other object of suitable material. In order to be readily attachable, an adhesive powder is applied onto the coated area after the final layer of ink has been applied. This adhesive can be adapted to be heat or pressure activated, and therefore makes it possible to attach the transfer in a simple manner. Other ways of activating the adhesive include microwave energy.
In other printing operations, for example offset printing of Christmas cards, a printed area is often coated with different types of powders, for example a glittery powder to resemble snow.
According to one technique,ithe application of coating powder is accomplished by dipping the screen or offset printed sheet in powder and then shaking off excess powder. This process is commonly referred to as xe2x80x9cdip""n""shakexe2x80x9d.
In other conventional devices, the powder is applied and excess removed in an automatic process. First the powder is applied in excess directly on the sheet, situated on a conveyor, i.e. a belt conveyor. Then, the removal of excess powder is provided through a suction device, working as a vacuum cleaner to remove powder not sticking to the coated area. In order to prevent the base sheet from leaving the conveyer and being pulled towards the suction device, a series of bars provided with rollers are arranged to extend across the conveyor. These rollers hold the sheet down as it passes under the suction device.
However, as it is necessary that the rollers make physical contact with the sheet, they also affect the coating process. At least two problems are prevalent: firstly the rollers risk damaging the colored layers that previously have been applied to the base sheet, by applying a too strong force to the sheet. Secondly, powder particles that are captured underneath the rollers are not subjected to the same suction force as the adjacent powder particles, and will therefore be removed to a lesser extent. Additionally, these powder particles will be pushed into and buried in the color coating.
These factors result in an uneven application of powder, typically with a striped pattern from the rollers.
Another problem with the above application technique, is that the final colored layer must not be too wet. Wet ink would stick to the rollers and cause stripes of ink over the entire sheet. After the final colored coating is applied, the sheet must therefore be allowed to dry, taking up valuable time and delaying the process.
Normally, a PVC-based or plastisole ink or is used, which first is allowed to dry and then is reheated and partially melted, in order to be sufficiently tacky for the powder particles to stick to the surface. A water-based ink is difficult to use, as such ink either is wet or dry, and only with difficulty can be made to reach such a tacky condition.
The overall object of the present invention is to provide an application of coating powder in a screen or offset printing process, overcoming the above-mentioned problems.
A specific object of the present invention is to assure a satisfying application of adhesive powder in a screen transfer printing process.
A further object of the present invention is to allow for use of water-based ink in a screen transfer printing process.
Yet another object of the present invention is to avoid the need of rollers retaining a sheet to be coated onto a conveyor.
These and other objects are accomplished by a device according to claim 1 and by a method according to claim 7.
The main feature of the device and the method according to the invention is that a jet of air is used as an air-knife, in order to remove excess powder from the base sheet. As the sheet is kept in close contact with the conveyor by suction, the sheet itself is essentially not moved at all by the air jet.
The two air flows being used, namely the suction of the base sheet towards the conveyor, and the air jet directed towards the sheet surface, may be arranged to cooperate and be connected in a cycle. The same powering device, e.g. a fan, can then be used to power the suction and the air-knife.
As suction is provided beneath the base sheet, the excess powder removed by the air-knife may readily be collected with the aid of this suction. The powder so collected may be transported back to the powder-applying device and be reused.